Monomer compound, graft copolymer compound, production method thereof, polymer electrolyte membrane, and fuel cell

ABSTRACT

A novel polymer electrolyte is provided that enables a solid polymer electrolyte used in fuel cells, for example, to have sufficient proton conductivity even in a low-water-content state or a zero-water-content state by using a monomer compound represented by the general formula (1), and a graft copolymer compound in which the monomer compound represented by the general formula (1) is graft-copolymerized to the main chain of a fluorine-containing hydrocarbon polymer.  
                 
 
     Tf indicates a trifluoromethane sulfonyl group (—SO 2 CF 3 ).

TECHNICAL FIELD

The present invention relates to a monomer compound comprising a superstrong-acidic group, a graft copolymer compound comprising the monomercompound, a production method thereof, a polymer electrolyte membranecomprising a super strong-acidic group, and a polymer electrolyte fuelcell comprising the polymer electrolyte membrane as a solid polymerelectrolyte membrane.

BACKGROUND ART

Fuel cells are devices for gaining electric energy by an operationalprinciple based upon reverse action of electrolysis of water. Ingeneral, hydrogen gained by transforming fuel such as natural gas,methanol, and coal, and oxygen in the air are supplied to the fuel cellsin order to obtain direct-current power while generating water. Becausethe fuel cells have a high efficiency of electric power generation andare capable of supplying clean energy, fuel cell power generation hasattracted attention.

The fuel cells are classified into a phosphoric acid type, a moltencarbonate type, a solid oxide type, and a solid polymer type, forexample, depending on the type of electrolyte used. In particular,polymer electrolyte fuel cells in which an ion exchange membrane (solidpolymer electrolyte membrane) is used as an electrolyte are advantageousin that they are substantially exclusively composed of solid cells andare therefore not subject to the problem of scattering of theelectrolytes or the maintenance thereof, the fuel cells can operate atlow temperatures of not more than 100° C., the start-up time isextremely brief, and they are capable of achieving high energy densityand reduction in size and weight, for example.

Therefore, polymer electrolyte fuel cells are being developed as powersources for automobiles, dispersed-type power sources for homes andbuildings, power sources for space vehicles, and portable power sources.Specifically, in terms of environmental issues such as global warmingand measures to reduce exhaust gas of automobiles, polymer electrolytefuel cells are gaining attention as fuel cells to be used forautomobiles.

Solid polymer electrolytes constitute a solid polymer material having anelectrolyte group such as a sulfonic group in a polymer chain. As thesolid polymer electrolytes have the property to strongly bind tospecific ions and to allow positive or negative ions to be selectivelytransmitted, they are formed as particles, fibers, or membranes and areused for various applications such as electrodialysis, diffusiondialysis, and battery diaphragms.

For example, a polymer electrolyte fuel cell comprises aproton-conductive solid polymer electrolyte membrane with a pair ofelectrodes provided with one electrode on each side thereof. Hydrogengas gained by reforming low molecular weight hydrocarbon such as methaneand methanol is supplied to one of the electrodes (fuel electrode) asfuel gas, and oxygen gas or air is supplied to the other electrode (airelectrode) as an oxidant in order to obtain an electromotive force.Water electrolysis is a method for producing hydrogen and oxygen byelectrolyzing water using a solid polymer electrolyte membrane.

In consideration of the application of the polymer electrolyte fuelcells to electric automobiles, it is desired that the operationtemperature of a fuel cell system be not less than 100° C. fordownsizing the cooling system and improving the CO tolerance and theefficiency of the electrodecatalyst. At such high temperatures, thevapor pressure of water increases, so that if the internal pressure ofthe batteries is to exist at a realistic level, the relative humidity ofambient atmosphere declines, making it necessary for the electrolytemembrane to have a sufficient proton conductivity in a low humidityenvironment.

In addition, although there is a demand that humidification from theoutside using pure water be eliminated in order to simplify the systemand avoid the problem of freezing in winter, if humidification iseliminated, it would require the ambient atmosphere inside the fuelcells to be maintained in a humid state using only generated water,resulting in low humidity environment likewise.

However, in general, the polymer electrolyte fuel cells are usuallyoperated at a temperature of not more than 100° C. This is becauseperfluoro electrolyte membranes such as Nafion (registered trademark ofDuPont) gains proton conductivity by containing water. Therefore, thewater content of the membrane (the water content relative to the weightof dried membrane) is an extremely important factor. The membrane mustbe maintained in a sufficiently water-containing state if it is toproduce proton conductivity, so that water control is required.Consequently, in general, reactant gas must be humidified when operatingthe batteries. However, humidification of the membrane becomesinsufficient at high temperatures of not less than 100° C., so that theproton conductivity declines.

Also, the perfluoro electrolyte membranes are difficult to manufacture,and are highly expensive. This makes it difficult to apply the perfluoroelectrolyte membranes to consumer use such as polymer electrolyte fuelcells as low-pollution power sources for automobiles

As mentioned above, perfluoro electrolyte membranes such as Nafioncannot maintain strength at high temperatures or sufficient conductivityin a high temperature and low humidity environment, so that it isdifficult to operate fuel cells in high temperature and low humidityconditions. Moreover, the cost is inevitably high.

In order to realize a fuel cell system capable of stable operation undersuch conditions as high temperature or a lack of humidification, it isextremely important to realize an electrolyte that exhibits conductivityin a low humidity environment. However, there has been no electrolytehaving a high ion-exchange capacity and capable of a high degree ofdissociation in a low humidity environment while providing a practicalstrength, and exhibiting sufficient proton conductivity in a hightemperature and low humidity environment.

JP Patent Publication (Kokai) No. 7-90111 A (1995) 1 discloses aninvention in which a metal catalyst and metal oxide are included in anelectrolyte membrane in order to provide a polymer solid electrolytecomposition having oxidation resistance equal to or greater than that ofa fluorine electrolyte, or sufficient in practical use, which issuperior in ion conductivity and the effect of crossover inhibition byhaving the ability of self generation and retainment of water, and whichis most suitable as a membrane for an electrochemical cell such as apolymer solid electrolyte fuel cell. Specifically, at least one metalcatalyst selected from the group consisting of platinum, gold,palladium, rubidium, iridium, and ruthenium is included in a polymersolid electrolyte selected from the group consisting of cation exchangeresins and/or anion exchange resins, and microscopic particles and/orfibers of metal oxide such as silica and titania are further included.

DISCLOSURE OF THE INVENTION

In the invention disclosed in JP Patent Publication (Kokai) No. 7-90111A (1995), in which the metal catalyst and metal oxide are included inthe electrolyte membrane, membrane functions such as the protonconductivity decline since substances that are essentially unnecessaryin terms of the membrane functions are added. This is caused bymaintaining the humidity using additives.

In view of such problems as mentioned above, it is an object of thepresent invention to obtain a novel polymer electrolyte for fuel cells,for example, having a sufficient proton conductivity even in alow-water-content state or zero-water-content state. Also, the presentinvention provides a polymer electrolyte fuel cell comprising a polymerelectrolyte membrane having such superior characteristics.

The inventors of the invention, as a result of extensive research,arrived at the present invention by solving the aforementioned problemsusing a graft copolymer compound comprising a monomer compound ofspecific structure in a graft chain thereof, the monomer compound havinga super strong-acidic group.

In a first aspect, the present invention is an invention of a monomercompound, which is represented by the following general formula (1).

The monomer compound of the present invention comprises Tf(trifluoromethane sulfonyl group (—SO₂CF₃)), which is a superstrong-acidic group.

In a second aspect, the present invention is an invention of a graftcopolymer compound comprising a super strong-acidic group, in which themonomer compound represented by the aforementioned general formula (1)is graft-copolymerized to the main chain of a fluorine-containinghydrocarbon polymer. The main chain of a fluorine-containing hydrocarbonpolymer is preferably an ethylene-tetrafluoroethylene copolymer, forexample. The graft copolymer compound is represented by the followinggeneral formula (2). Tf indicates a trifluoromethane sulfonyl group(—SO₂CF₃).

In a third aspect, the present invention is an invention of a method formanufacturing a graft copolymer compound, the method comprising causingthe monomer compound represented by the aforementioned general formula(1) to be graft-copolymerized to a fluorine-containing hydrocarbonpolymer.

In a fourth aspect, the present invention is an invention of a polymerelectrolyte membrane obtained by processing the aforementioned graftcopolymer compound. The invention also provides a polymer electrolytemembrane in which the monomer compound represented by the aforementionedgeneral formula (1) is graft-copolymerized to a base film comprising afluorine-containing hydrocarbon polymer. The polymer electrolytemembrane according to the present invention shows a sufficient protonconductivity even in a low-water-content state or zero-water-contentstate.

In a fifth aspect, the present invention is an invention of a polymerelectrolyte fuel cell comprising the aforementioned electrolytemembrane, reactive poles that sandwich the electrolyte membrane, andseparators that sandwich the reactive electrodes.

By using the monomer compound comprising a super strong-acidic group asmentioned above, a polymer electrolyte membrane can be obtained showinga sufficient proton conductivity even in a low-water-content state orzero-water-content state, without using additives unnecessary for themembrane components. By using the polymer electrolyte membrane in a fuelcell, the system operation temperature can be increased and a humidifiercan be eliminated. As a result, the fuel cell system can be reduced insize, the CO tolerance of electrocatalyst can be improved, and freezecan be prevented, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an introduction scheme of a super strong-acidic group by acomparative example (conventional method).

BEST MODE FOR CARRYING-OUT OF THE INVENTION

In the following, the details of carrying out the embodiments of thepresent invention are described.

A monomer compound comprising a super strong-acidic group represented bythe aforementioned general formula (1) is synthesized in the followingscheme, for example.

Examples of a fluorine-containing hydrocarbon polymer which constitutesthe main chain of a graft copolymer compound comprising a superstrong-acidic group according to the present invention includeethylene-tetrafluoroethylene copolymer, polytetrafluoroethylene,polyvinylidene fluoride, and hexafluoropropylene-tetrafluoroethylenecopolymer, for example.

The manner of graft copolymerization is not especially limited. Forexample, a monomer compound represented by the aforementioned generalformula (1), which constitutes a side chain, is graft-copolymerized inthe presence of the fluorine-containing hydrocarbon polymer, whichbecomes the main chain, using thermopolymerization, radiation-inducedpolymerization, or a radical initiator.

Also, the manner of graft copolymerization in which the monomer compoundrepresented by the aforementioned general formula (1) isgraft-copolymerized to a base film comprising a fluorine-containinghydrocarbon polymer is not especially limited. For example, a monomercompound represented by the aforementioned general formula (1), whichconstitutes a side chain, is graft-copolymerized in the presence of thebase film comprising the fluorine-containing hydrocarbon polymer, whichconstitutes the main chain, using thermopolymerization, radiationinduced polymerization, or a radical initiator.

The obtained graft copolymer compound can be processed to manufacture amembrane by dissolving the graft copolymer in a solvent, casting theobtained solution on a support such as a glass plate, and then drying.The obtained film is, if necessary, processed by a hydrochloric acidsolution or nitric acid solution, and then rinsed sufficiently withion-exchanged water.

Examples of the solvent include an aromatic hydrocarbon solvent, anether solvent, a ketone solvent, an amide solvent, a sulfone solvent, ora sulfoxide solvent. The sulfoxide solvent and the amide solvent arepreferable among these solvents because of high solubility. Thesulfoxide solvent is preferably dimethyl sulfoxide, for example. Theamide solvent is preferably N,N-dimethylformamide,N,N-dimethylacetamide, or N-methyl-pyrrolidone, for example.

In order to improve the mechanical strength of the electrolyte membraneof the present invention, the electrolyte membrane may be irradiatedwith an electron beam or a radiation beam for crosslinking. It may alsobe impregnated in a porous film or sheet to make a complex, and fiber orpulp may be mixed to reinforce the film. Although the thickness of theelectrolyte membrane is not especially limited, a thickness of 10 to 200μm is preferable. A membrane whose thickness is less than 10 μm tends tohave a declined strength, and a membrane whose thickness is greater than200 μm tends to lack the characteristics of an electrochemical devicebecause of increased membrane resistance. The membrane thickness can becontrolled by the concentration of the solution or the thickness of thecoating on the substrate.

The polymer electrolyte membrane according to the present invention canprovide a sufficient proton conductivity even in a low-water-contentstate or a zero-water-content state without using additives unnecessaryfor the membrane components.

Therefore, the polymer electrolyte membrane according to the presentinvention, comprising a super strong-acidic group, is suitable as an ionexchange membrane in a polymer electrolyte fuel cell.

In the following, a fuel cell according to the present invention isdescribed. The fuel cell according to the present invention can bemanufactured by joining conductive substances as a catalyst and acollector to both surfaces of the polymer electrolyte membrane for fuelcells as mentioned above. The catalyst is not especially limited andvarious well-known substances can be used as long as they can activatean oxidation-reduction reaction with hydrogen or oxygen. However, fineparticles of platinum are preferable. Fine particles of platinum areoften carried by particulate or fibrous carbon, such as activated carbonor graphite. Regarding the conductive substance as a collector, althoughvarious well-known materials can be used, porous carbon non-woven fabricor carbon paper is preferable for efficiently transporting source gas tothe catalyst. For joining the platinum fine particles or the carboncarrying the platinum fine particles to the porous carbon non-wovenfabric or the carbon paper, and joining it to the polymer electrolytemembrane, well-known methods can be used.

Specifically, the fuel cell according to the present invention is PEFC.A stack formed by piling up a plurality of fuel cells is used as thefuel cell. And the aforementioned electrolyte membrane according to thepresent invention is used as the electrolyte membrane. A gas-supplyingdevice that individually supplies fuel gas and oxidizing gas isconnected to reactive electrodes on both sides that sandwich theelectrolyte membrane via separators on the corresponding sides thereof.The fuel gas is preferably hydrogen gas, and the oxidizing gas ispreferably air or oxygen gas.

The fuel cells according to the present invention have a structure wherethe electrolyte membrane is sandwiched on both sides thereof by thereactive electrodes, and an MEA sandwiched by diffusion layers issandwiched on both sides thereof by separators. The reactive electrodesare not especially limited and conventional electrodes can be used. Forexample, a catalyst in which platinum or platinum alloy is dispersed oncarbon powders can be used. The reactive electrodes can be formed byprocessing a membrane on the surface of the electrolyte membrane usingthe catalyst as it is or after mixing it with a binder, for example,such as an electrolyte solution of the invention. As the diffusionlayers, a mixture of conventional carbon powders and hydrophobic polymerpowders can be used, for example. The diffusion layers can also beformed including the electrolyte solution of the present invention. Theseparators may also employ conventional materials and forms. Theseparators are formed with a channel to which the gas-supplying devicethat supplies reactant gas and means for removing non-reacted reactantgas and generated water are connected.

The polymer electrolyte membrane according to the present inventionshows sufficient proton conductivity even in a low-water-content stateor zero-water-content state. Thus, in the fuel cell (PEFC) according tothe present invention, system operation temperature can be increased andthe need for a humidifier can be eliminated.

EXAMPLES

In the following, the present invention is described more specificallywith reference to examples and comparative examples.

EXAMPLES Monomer Synthesis

A monomer comprising a super strong-acidic group according to thepresent invention was synthesized by conducting the following reactionin Benzene/THF=1:1. The reaction temperatures and reaction time ware at0° C. to room temperature, for 0.5 hour, and then at 70° C., for 6hours.

[Manufacturing of Graft Copolymer/Electrolyte Membrane]

The obtained monomer comprising a super strong-acidic group was directlygraft-copolymerized to a base film comprising anethylene-tetrafluoroethylene copolymer.

Comparative Example

Tf (trifluoromethane sulfonyl group (—SO₂CF₃)) as a super strong-acidicgroup was introduced to an ethylene-tetrafluoroethylene copolymer by thescheme shown in FIG. 1 in the same manner as in the present invention.

[Amount of Introduction of Super Strong-Acidic Group]

The amount of introduction of super strong-acidic groups per gram of theelectrolyte membranes was compared between the example and thecomparative example. Table 1 shows the results. TABLE 1 Amount ofintroduction [mmol/g] Example 4.85 Comparative example 0.23

As the result of Table 1 shows, about 20 times more introduction wasconfirmed in the present invention than in the comparative example. Thisis due to the fact that, since in the comparative example, thelithiation of the Br group does not proceed sufficiently in the firsthalf of the reaction step, resulting in a small amount of reaction fieldin the latter half such that the amount of introduction of the intendedsuper strong-acidic group cannot be increased.

[Conductivity]

The conductivity of the electrolyte membranes after introduction of thesuper strong-acidic groups was compared between the example and thecomparative example. Table 2 shows the results. The measurement of theproton conductivity was conducted by cutting the membranes of theexample and comparative example into a 5×40 mm section, and measuringthe alternating-current impedance via the four terminal method. Themeasurement was conducted under conditions including temperatures of 80°C. and 120° C. with relative humidity of 100%, a constant current valueof 0.005 mA, and the sweep frequency of 10 to 20000 Hz. The conductivitywas measured using the obtained impedance and the distance between themembrane and the electrodes. TABLE 2 Conductivity [S/cm²] Example 3.6 ×10⁻³ Comparative example 2.8 × 10⁻⁵

As the results of Table 2 show, the amount of introduction of superstrong-acidic groups is larger in the present invention.

INDUSTRIAL APPLICABILITY

The highly proton-conductive electrolyte of the present invention canprovide a sufficient proton conductivity with a high degree ofdissociation even in a low humidity environment since a superstrong-acidic group is used as for ion exchange. When the electrolyte ofthe graft copolymer according to the present invention is suitably used,for example, for the solid polymer electrolyte membrane of a polymerelectrolyte fuel cell, power generation can be stably performed even ina low humidity environment. As a result, the fuel cell can be operatedwithout humidification and at high temperatures, thereby allowing thebattery to be reduced in size, provided with an anti-freeze property,and improved in efficiency, for example. The present invention willcontribute a great deal to the industry.

Also, the highly proton-conductive electrolyte of the present inventioncan be suitably used for water electrolysis, common salt electrolysis,oxygen thickeners, humidity sensors, and gas sensors, for example, inaddition to fuel cells.

1. A monomer compound represented by the general formula (1): wherein Tfindicates a trifluoromethane sulfonyl group (—SO₂CF₃).


2. A graft copolymer compound in which the monomer compound representedby the general formula (1):

is graft-copolymerized to the main chain of a fluorine-containinghydrocarbon polymer, wherein Tf indicates a trifluoromethane sulfonylgroup (—SO₂CF₃).
 3. The graft copolymer compound according to claim 2represented by the general formula (2):

wherein the main chain of said fluorine-containing hydrocarbon polymeris an ethylene-tetrafluoroethylene copolymer, and Tf indicates atrifluoromethane sulfonyl group (—SO₂CF₃), n is not less than 10, and mis not less than
 3. 4. A method for manufacturing a graft copolymercompound comprising graft-copolymerizing the monomer compoundrepresented by the general formula (1):

to a fluorine-containing hydrocarbon polymer compound, wherein Tfindicates a trifluoromethane sulfonyl group (—SO₂CF₃).
 5. A polymerelectrolyte membrane wherein the graft copolymer compound according toclaim 2 is processed into a membrane.
 6. A polymer electrolyte membranewherein the monomer compound represented by the general formula (1):

is graft-copolymerized to a base film comprising a fluorine-containinghydrocarbon polymer, wherein Tf indicates a trifluoromethane sulfonylgroup (—SO₂CF₃).
 7. A polymer electrolyte fuel cell comprising theelectrolyte membrane according to claim 5, reactive poles that sandwichsaid electrolyte membrane on both sides thereof, and separators thatsandwich said reactive poles.
 8. A polymer electrolyte membrane whereinthe graft copolymer compound according to claim 3 is processed into amembrane.
 9. A polymer electrolyte fuel cell comprising the electrolytemembrane according to claim 6, reactive poles that sandwich saidelectrolyte membrane on both sides thereof, and separators that sandwichsaid reactive poles.